Three more health workers vaccinating children against polio have been shot dead in Pakistan in attacks blamed on Islamic militants, bringing the total killed this week to eight.

Wednesday's attacks all took place in the restive western frontier province of Khyber Pakhtunkhwa – one just outside the city of Peshawar and two others in the town of Charsadda. Two men and a woman have been killed.

The volunteers were taking part in a three-day government-led drive, supported by the World Health Organisation and Unicef, to vaccinate tens of millions of children at risk from polio in Pakistan.

After a decades-long struggle by multilateral organisations, governments and NGOs worldwide, the disease is now endemic only in three countries: Pakistan, Afghanistan and Nigeria.

It was not clear who was behind the shootings but Taliban insurgents have repeatedly denounced the anti-polio campaign as a western plot. Relatives of those shot earlier this week said several of the victims had received death threats in recent days.

Some confusion has emerged about whether and to what extent the anti-polio drive has been halted after a security meeting between officials in the hours following Tuesday's killings.

The United Nations in Pakistan has pulled all staff involved in the campaign off the streets, Michael Coleman, a spokesman, said. However, the Pakistani government said immunisation had continued in some areas without UN support, although many workers refused to go out.

Women health workers held protests in Karachi and the capital, Islamabad. "We go out and risk our lives to save other people's children from being permanently handicapped, for what? So that our own children become orphans?" Ambreen Bibi, a health worker, said at the Islamabad protest.

Government officials admit they have been caught off guard by the violence, saying they had not foreseen attacks in areas far from the Taliban strongholds in the north-west of the country. "We didn't expect such attacks in Karachi," said Mustafa Nawaz Khokhar, minister for human rights, who oversees the polio campaign.

Some Islamists and Muslim preachers in Pakistan say the polio vaccine is a western plot to sterilise Muslims to stop population growth. Other religious leaders have tried to counter that myth.

However there has been a severe backlash against immunisation for polio and other diseases in Pakistan since the CIA used a local doctor to set up a fake vaccination programme as the agency closed in on Osama bin Laden in his hiding place in the northern town of Abbottabad last year.

Statistics released in October showed an improvement in the polio situation in Pakistan, with 47 children paralysed in 27 districts compared with 154 in 48 districts in 2011. However, in 2005 only 28 new cases were registered.

Officials and campaigners say there is reason to be optimistic that polio can be eradicated in Pakistan if a "final push" can be made. In neighbouring India, a mass vaccination campaign involving more than a million volunteers reduced cases nationally from 741 to 42 between 2009 and 2010, and down to a single case last year.